reference to :http://bbs.9ria.com/thread-253058-1-1.html
在android SDK的framework/base/cmds目录下了，有不少目录，这些目的最终都是build出一个bin文件，再存放到/system/bin目 录下，对于C/CPP写的命令，我们还是比较好理解的，都有一个main函数作为入口，但是在cmds目录下还有一些原生代码是java的，比如 input、settings，那么这种类型的命令是怎么实现的呢?
笔者研习了原生的命令实现，写了一个demo，抛砖引玉吧!暂时叫strong吧!我们都知道java写的文件最后都是编译成了class文
件，java类里面也有很多接口，在android平台上cmds目录下的各模块的java文件都实现了一个共同的方法，还是叫main()，真是情有独
钟啊!当然从技术角度看叫其他名字也是可以的。那我们就简单实现以下这个class吧!如下：
java代码
public final class strongcmd {
static final String TAG = "strong";
static String[] mArgs;
int mNextArg;
public static void main(String[] args) {
printUsage();
System.err.println("Wellcom strong test function!!");
try {
new strongcmd().run();
} catch (Exception e) {
System.err.println("Unable to run settings command");
}
}
public void run() {
try {
System.err.println("Now strong run() again");
} catch (Exception e) {
System.err.println("Now strong run() Exception");
}
}
private static void printUsage() {
System.err.println("usage: strong -a -b -h");
System.err.println("'a' is for add");
System.err.println("-h for help");
}
}
写好Android.mk，编译好这个文件，会生成strong.jar包，包含这个class。那么，又怎么跟命令挂钩呢?先看看Android.mk，如下：
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES := $(call all-subdir-java-files)
LOCAL_MODULE := strong
LOCAL_MODULE_TAGS := optional
include $(BUILD_JAVA_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := strong
LOCAL_SRC_FILES := pre_strong
LOCAL_MODULE_CLASS := EXECUTABLES
LOCAL_MODULE_TAGS := optional
include $(BUILD_PREBUILT)
上一部分是BUILD_JAVA_LIBRARY，关键在下面，利用的是BUILD_PREBUILT，添加一个预编译好的应用程序，我们叫pre_strong，它有可执行的权限，看看它的具体实现吧!
base=/system
export CLASSPATH=$base/framework/strong.jar
exec app_process $base/bin com.android.commands.strong.strongcmd "$@"
首先还是设置好这个java lib的路径，如何再调用app_process来执行，主要是把这个类名要给对，app_process其实也是个命令。在app_process里 面，还是一样利用JNI技术，在java ENV里面查找传给app_process的class，找到这个class后再去找main函数接口的field，然后再call这个main接口，这 样就call到java里面去了。
下面简要看看app_process的关键代码吧!
virtual void onVmCreated(JNIEnv*env) {
if (mClassName == NULL) {
return; // Zygote. Nothing to do here.
}
/*
* This is a little awkward because the JNI FindClass call uses the
* class loader associated with the native method we're executing in.
* If called in onStarted (from RuntimeInit.finishInit because we're
* launching "am", for example), FindClass would see that we're calling
* from a boot class' native method, and so wouldn't look for the class
* we're trying to look up in CLASSPATH. Unfortunately it needs to,
* because the "am" classes are not boot classes.
*
* The easiest fix is to call FindClass here, early on before we start
* executing boot class Java code and thereby deny ourselves access to
* non-boot classes.
*/
char*slashClassName = toSlashClassName(mClassName);
mClass = env -> FindClass(slashClassName);
if (mClass == NULL) {
ALOGE("ERROR: could not find class '%s'\n", mClassName);
}
free(slashClassName);
mClass = reinterpret_cast(env -> NewGlobalRef(mClass));
}
virtual void onStarted() {
sp proc = ProcessState::self ();
ALOGV("App process: starting thread pool.\n");
proc -> startThreadPool();
AndroidRuntime * ar = AndroidRuntime::getRuntime ();
ar -> callMain(mClassName, mClass, mArgC, mArgV);
IPCThreadState::self () -> stopProcess();
}
className) {
// Remainder of args get passed to startup class main()
runtime.mClassName = className;
runtime.mArgC = argc - i;
runtime.mArgV = argv + i;
runtime.start("com.android.internal.os.RuntimeInit",
application ? "application" : "tool");
}
Android平台提供的app_process，还是相当不错的，比较实用，利用好app_process还是可以写成很多供我们自己开发、测试、定制一些特殊的程序，给开发带来了很大的便利。